The present invention generally relates to an apparatus for determining the content of a stable element in a sample volume using X-ray fluorescence. The elements concerned are those having atomic numbers (atomic numbers in the periodic system) higher than 47. The invention particularly relates to the determination in vivo of the concentration of iodic X-ray contrast agents.
By comparative tests, it has been established that X-ray fluorescence technology used for quantitative measurements in vivo of the concentration of iodic X-ray contrast agents in well blood-filled soft tissue, may provide a measurement of the body's rate of excreting the contrast agent. This excretion rate reflects in its turn the function of the kidneys. Thus, the renal function may be determined by repeated measurements of the concentration of the contrast agent in the soft tissue.
In such X-ray fluorescence analysis, use has been made of int.al. .sup.241 Am as excitation radiation source, and a Ge(Li) detector has been employed for registering the characteristic K.sub..alpha. X-ray radiation from iodine. This employed detector, however, is difficult to operate, bulky and expensive and moreover requires low prevalent temperatures. Therefore, it is not suited for use in a mass-produced apparatus for carrying out large-scale measurements. It would be desirable to be able to employ, for example, an NaI(Tl) detector instead of the Ge(Li) detector. However, the NaI(Tl) detector suffers from the drawback that registration of the K.sub..alpha. X-ray radiation characteristic of iodine, which is emitted from the contrast agent will be overlaid by the registration of Compton-scattered photons in the sample, which interact with iodine atoms in the detector crystal proper.